A player with "superstar" potential will still cap once he's hit X number of training points. His maximum ability remains the same whether he's 19 or 29. The only difference is that the 19 y/o will train much, much better than the 29 y/o. You could theoretically train a 29 y/o "superstar" potential player to the same ceiling as a 19 y/o "superstar". The only impact age has is on training speed.